1. Field of the Invention
The present invention relates to control circuits with chopping means used in particular in automotive vehicles, for example to power window lifters, windscreen wipers, robotized gearboxes or steering column adjustment systems.
2. Description of the Related Art
Represented in FIG. 1 is such a circuit, here a pulse width modulation (PWM) circuit which conventionally includes a source 5 delivering a supply voltage V.sub.bat, followed by a low-pass filter 10 and then a chopping circuit 15 with two outputs A and B, and filtering circuits F1, F2 installed at these two outputs A and B. A load CH has its terminals connected at the output of the filters F1 and F2.
The chopping circuit 15 (or converter), typically a bridge of circuit breakers forming a chopping type bidirectional voltage source (here an H bridge), delivers two signals with variable pulse width on the two outputs A and B.
The role of the filtering circuits F1 and F2 is to reduce the electromagnetic disturbances (EMC) generated by the chopping (chopping noise), especially when the load CH is far from the chopping circuit 15 (this same type of filter is also used at the input of converters).
The load CH is traversed by a current I (load current) which is substantially continuous although it can change direction in certain applications, as a function of the sign of a control voltage.
According to a first conventional example, F1 and F2 are both formed by an LC circuit (such as that represented in FIG. 2 with reference to F1), whose coil L1 links an output of the power circuit 15 to a terminal of the load CH, and whose capacitor C1 links this same terminal of the load CH to ground.
These known circuits remain rather unsatisfactory, in particular when the load CH is to be supplied in both directions, such as for example for electric motors of certain applications, since the load current I may reach a high enough value to saturate the coil L1, which then exhibits decreased inductance, denying the LC circuit its role as filter. Particularly significant electromagnetic disturbances are then obtained at the terminals of the load CH.
Moreover, the capacitor C1 is then linked directly to the output of the circuit 15, reducing the efficiency of the latter, and, because the chopped voltage exhibits high spans and low spans of different durations, the capacitor C1 can charge up to its maximum capacitance.
For this reason, it is often necessary to limit the capacitance of the capacitor C1. However, in this case the filter exhibits a high impedance, and, because it is in series with the load CH, it forms a voltage divider. Consequently, here again the converter 15 generates electromagnetic disturbances at the terminals of the load CH.
It has indeed been proposed that coils having a higher inductance be installed, but they are expensive and bulky and dissipate heat.
The inductance can reasonably reach 10 .mu.H with a saturation current of 15 to 20 A and the capacitance may reach a value of 20 .mu.F maximum.
It has been proposed (FIG. 3) that an LC circuit be arranged at the output of each of the two branches of such a chopping circuit and that the coils of the two circuits be wound around the same core.
Each coil is traversed by a current whose average is substantially the load current. The coils therefore induce average magnetic fields having opposite directions and substantially equal values, regardless of the direction of the load current.
The magnetic fields therefore compensate one another, thereby giving a zero mean flux and protecting the coil from saturation. This type of circuit applies in particular when the load is inductive, and the common core used is for example a toroidal core. In such a device, a capacitor is necessarily installed on each side of the load.
Such a device requires complex adaptations and a large number of components. It is therefore still fairly expensive.